Two-dimensional electron gas systems such as quantum wells exhibit quantized electronic states in one dimension and have a step-like density of states. The charge carriers are thus localized in one dimension (i.e., growth direction) and can freely move in the in-plane directions. See U.S. Pat. No. 5,442,221, incorporated herein by reference in its entirety.
Localized charge carriers exhibit high in-plane mobility in a wide charge carrier concentration range, which can be precisely controlled by conventional epitaxial crystal growth. Carrier mobility is limited by carrier scattering mechanisms that are typically dominated by optical phonon scattering, ionized impurity scattering and alloy scattering when alloys containing more than 2 atoms are used—i.e., ternary (3 atoms), quaternary (4 atoms) and quinternary (5 atoms) alloys. See M. Hayne et al. “Remote impurity scattering in modulation-doped GaAs/AlxGa1-xAs heterojunctions”, Phys. Rev. B., Vol. 57, No. 23, 1998; and A. K. Saxena, A. R. Adams, “Determination of alloy scattering potential in Ga1-xAlxAs alloys,” J. Appl. Phys., Vol. 58, 2640, 1985, incorporated herein by reference in their entireties.
Increasing carrier mobilities is a key challenge in semiconductor device fabrication processes. See, e.g., K.-J. Friedland, R. Hey, H. Kostial, R. Klann, and K. Ploog, “New Concept for the reduction of impurity scattering in remotely doped GaAs quantum wells,” Phys. Rev. Lett., Vol. 77, No. 22, 1996, and U.S. Pat. Nos. 4,912,451; 5,657,189; 7,388,235; and 6,316,124, incorporated herein by reference in their entireties.